Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
Disables the internal PDF viewer in Microsoft Edge. If you enable this policy Microsoft Edge treats PDF files as downloads and lets users open them with the default application. If Microsoft Edge is the default PDF reader, PDF files aren't downloaded and will continue to open in Microsoft Edge. If you don't configure this policy or disable it, Microsoft Edge will open PDF files (unless the user disables it).